1. Field of the Invention
The present invention generally relates to satellite position location systems and, more particularly, to a method and apparatus for processing location service messages in a satellite position location system.
2. Description of the Background Art
Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver.
More specifically, each GPS signal available for commercial use utilizes a direct sequence spreading signal defined by a unique pseudo-random noise (PN) code (referred to as the coarse acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carrier signal (referred to as the L1 carrier) and uniquely identifies a particular satellite. The PN code sequence length is 1023 chips, corresponding to a one millisecond time period. One cycle of 1023 chips is called a PN frame or epoch.
GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences. These measured time delays are referred to as “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. A set of four pseudoranges together with a knowledge of absolute times of transmission of the GPS signals and satellite positions in relation to these absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission (or reception) are needed in order to determine the positions of the GPS satellites at the times of transmission and hence to compute the position of the GPS receiver. The satellite positions may be obtained from satellite trajectory data broadcast by the satellites, referred to as ephemeris.
In some GPS applications, the signal strengths of the satellite signals are so low that either the received signals cannot be processed, or the time required to process the signals is excessive. As such, to improve the signal processing, a GPS receiver may receive assistance data from a network to assist in satellite signal acquisition and/or processing. For example, the GPS receiver may be integrated within a cellular telephone and may receive the assistance data from a server using a wireless communication network. This technique of providing assistance data to a remote receiver has become known as “Assisted-GPS” or A-GPS.
In some A-GPS systems, data related to location services is communicated between the server and the A-GPS receiver over a control channel of the wireless communication network (“control-plane (C-plane) signaling”). C-plane signaling, however, requires the development of an extensive network infrastructure. In addition, cellular network operators are hesitant to actively employ C-plane signaling due to the low number of A-GPS handsets currently in use.
Recently, the wireless industry has strongly considered the use of an alternative protocol for messaging. Specifically, the industry has investigated the benefits of communicating location service data between the server and the A-GPS receiver over a network data link of the wireless communication network (“user-plane (U-plane) signaling”). For example, location service data may be communicated using an internet protocol (IP). Presently, a significant number of cellular phones are designed and manufactured with IP capabilities (e.g., General Packet Radio Service (GPRS) capabilities). Thus, U-plane signaling takes advantage of pre-existing network infrastructure and simply builds on top of this existing system. Consequently, changes to the base station infrastructure software, which are used to handle location service messages, can be avoided.
Presently, A-GPS devices are manufactured to support only one of C-plane signaling or U-plane signaling. If an A-GPS device that supports one type of signaling protocol enters the service area of a cellular network that supports the other type of signaling protocol, the A-GPS device cannot receive the necessary location service messages. Accordingly, there exists a need in the art for a method and apparatus capable of processing location service messages in a satellite position location system regardless of the use of C-plane signaling or U-plane signaling.